1. Field of the Disclosure
The present invention relates to quantum dot light emitting diode devices, and more particularly to a quantum dot light emitting diode device in which a hole transport layer is formed after forming a quantum dot light emitting layer by a solution process to form an inverted type quantum dot light emitting diode device for making free selection of a hole transport layer material that enables easy injection of a hole to the quantum dot light emitting layer; and a display device therewith.
2. Discussion of the Related Art
Importance of the display device is being further emphasized as a visual information transmission media in an information oriented society, and in order to occupy a major position in the future, the display device is required to meet requirements of being thinner and lighter, and having less power consumption and a better picture quality.
Of the display devices, recently, a quantum dot light emitting diode device is under research, which enables display by using a light emitting material, to fabricate a slim display device, and to drive for a long time, and has high color purity.
The quantum dot QD is a nano particle. The quantum dot with a diameter of a nanometer size emits a light as an unstable electron moves down from a conduction band to a valence band, wherein the wave length of the light from the quantum dot becomes the shorter as a size of a quantum dot particle is the smaller, and the longer as the size of the quantum dot particle is the greater. These are unique electric and optical characteristics which are different from present semiconductor material. Therefore, by controlling the size of the quantum dot, a visible light of a desired wave length can be produced, and by varying the size and composition of the quantum dot, a variety of colors can be produced.
In comparison to a general organic light emitting diode device, the quantum dot light emitting diode device is a display device which uses the quantum dot instead of an organic light emitting material as a material of the light emitting layer. Though an Organic Light Emitting Diode OLED which uses the organic light emitting material produces single color of white, red, blue or so on depending on a kind of the device, the organic light emitting diode has a limitation in expressing many color lights brilliantly. Opposite to this, since the quantum dot light emitting diode device can produce a desired natural color by controlling the size of the quantum dot, has a good color reproduction ratio, and brightness which does not fall behind the light emitting diode, the quantum dot light emitting diode device is spot lighted as a material which can supplement drawbacks of the light emitting diode LED which is paid attention as a next generation light source.
A structure of a general quantum dot light emitting diode device will be described.
FIGS. 1A and 1B illustrate a schematic section of a general quantum dot light emitting diode device and a diagram of a band gap energy thereof.
Referring to FIGS. 1A and 1B, a general quantum dot light emitting diode device is provided with an anode 10 and a cathode 50 opposite to each other on a substrate 100, a quantum dot light emitting layer 30 between the anode 10 and the cathode 50, a hole transport layer 20 between the anode and the quantum dot light emitting layer 30, and an electron transport layer 40 between the quantum dot light emitting layer 30 and the cathode 50.
The quantum dot light emitting layer 30 is filled with a plurality of quantum dots 31 each with a diameter of a nano size, formed, for an example, by coating a quantum dot solution having the quantum dots 31 dissolved in a solvent on the hole transport layer 20 by a solution process and volatilize the solvent.
The hole transport layer 20 makes injection of the hole from the anode 10 easy, and serves to transport the hole to the quantum dot light emitting layer 30.
The electron transport layer 40 makes injection of the electron from the cathode easy, and serves to transport the electron to the quantum dot light emitting layer.
The quantum dot light emitting layer 30 is formed by applying a quantum dot material by the solution process for receiving the hole from the hole transport layer 20 and the electron from the electron transport layer 40 and combining the hole and the electron to emit a light.
In this instance, each of the quantum dots 31 has a core 33 component at a center for emitting the light, a shell at a surface thereof for protection, and a ligand 35 component covering a shell surface for spreading the solvent.
In this instance, since the core 33, the shell 34, and the ligand 35 have band gap energy differences different from one another due to difference of components, such that the farther from the core 33, the greater the band gap energy difference. In this case, when the hole is transported from the hole transport layer 20 to the quantum dot 31, the shell 34 or the ligand 35 becomes to have an HOMO energy level lower than the HOMO energy level of the core 33, acting as an energy barrier at the time the hole is transported to the quantum dot. Alikely, at the time the electron is transported to the quantum dot light emitting layer 30 from the electron transport layer 40, an LUMO energy level of the shell 34 or the ligand 35c of the quantum dot becomes higher than the LUMO energy level of the core, making the shell 34 of the ligand to act as the energy barrier.
In the meantime, as shown in the drawings, it can be known that the energy barrier is very high at the time of hole transportation relative to transportation of the electron, because the HOMO energy level of the shell or ligand of the quantum dot light emitting layer 30 is very low in comparison to the high HOMO energy level of the hole transportation layer 20 at the time the hole is transported from the hole transport layer 20 to the quantum dot light emitting layer 30. Consequently, it can be foreseen that the hole injection into the quantum dot light emitting layer 30 is difficult to cause poor light emitting efficiency, to require a high driving voltage for making the quantum dot light emitting layer 30 to emit the light and to have poor light emitting efficiency.
Moreover, in a process the quantum dot light emitting layer 30 is formed on the hole transport layer 20 by the solution process, the solvent which is used for forming the quantum dot light emitting layer 30 causes to dissolve a component of the hole transport layer 20. Since formation of the hole transport layer 20 which will not be dissolved by the solvent is a key, presently materials for forming the hole transport layer 20 are limited.
As a spontaneous light emitting device, the related art organic light emitting layer has the following problems.
First, the great hole energy barrier between the hole transport layer and the quantum dot light emitting layer makes hole transportation to the quantum dot light emitting layer difficult, to require a high driving voltage, and to cause poor light emitting efficiency due to the difficulty in transportation of the hole.
Second, at the time of fabrication of the general quantum dot light emitting diode device, the quantum dot light emitting layer is formed on the hole transport layer by the solution process. Since, at the time the quantum dot light emitting layer is formed, the component of the hole transport layer under the quantum dot light emitting layer is also dissolved, to required to select a material that will not be dissolved in the solution process, hole transportation layer materials are limited.